[P39]: Full recovery of muscle function after chemical denervation with botulinum toxin of developing extraocular muscle

Strabismus is a misalignment of the visual axes, due to an imbalance of extraocular muscle function. Chemical denervation (that weakens a relatively too strong extraocular muscle) results in permanent improvement of strabismus after botulinum toxin treatment in developing extraocular muscle, but not in adult muscle (Porter et al., 1995), where muscle function fully recovers within months. The toxin may have permanent muscle-weakening effects in developing extraocular muscle, or the long-term effects may be due to changes in central adaptive mechanisms (Tychsen, 2005). Methods: To determine whether botulinum toxin permanently weakens developing extraocular muscles, we examined the chicken oculomotor system. Following injection of 0.5 ng botulinum toxin A or sham injection at posthatch day 1 (P1), we measured morphological and physiological parameters of superior oblique muscles from P2 to P112: muscle mass, average muscle fiber diameter, contractile force (twitch and tetanic tension induced by nerve stimulation in situ), neuromuscular junctions (identified by fluorescent label), and density of mitochondria (fractional area and number by electron miscroscopy). Results: Muscle mass and average muscle fiber diameter were not acutely affected. Muscle contractile force was transiently reduced, and recovered fully by P77-P112 (Fig. 1). Endplate structures were largely normal, with transient sprouting. Mitochondrial densities were transiently reduced, and recovered by P77–P112. All parameters examined thus show full recovery of muscle function and structure. Fig. 1.